I bought Ahrens's book to learn how to take smart notes. It should've been called Why to Take Smart Notes. The book was more about how good and lifechanging it is to use Zettelkasten, which was a bummer because I was already interested enough in the idea to buy a book about it. I was looking for more of a how-to.
I wrote a bit about using Obsidian with bottom-up / smart notes.
My Obsidian Vault setup: https://bryanhogan.com/blog/obsidian-vault
All posts about Obsidian: https://bryanhogan.com/tags/obsidian
Maybe this helps?
A few chapters in, this is a great how-to book: A System for Writing (Bob Doto)
https://writing.bobdoto.computer/zettelkasten/